The English monarchy invented the idea of the Vikings as a way to create a sense of fear and uncertainty among the population, making it easier to maintain control.
In fragments of records found it was shown that by the early 11th century that some Saxons claimed that they were really of Viking descent. This was used to bolster their own morale entering battle by using the mythical viking battle prowess to enhance the view that was already held of the power of Saxons.
On the other side, William the Conqueror in particular continued the myth of the vikings from the north to scare the populace into following him. Because of all of the stories they had been told over the centuries the populace completely believed that they had magic powers and great battle might. By showing that he was able to best even their mystic powers it increased his own personal glory.
Of course much of this went against church teachings so those rumors were entirely behind closed doors. However, it spread through the land of word of mouth. This was aided by a number of nobles who fought with him at the battle of Hastings spread tales claiming he personally stood against several “Viking Mystics”. By increasing his legends it increased their renown by association. It was all one long string of tales used to control the people.